Using v-erbB as a probe under reduced hybridization stringency, we have identified three exons of a novel erbB-related gene with closest identity of 64% and 67% to a contiguous region within the tyrosine kinase domains of the platelet-derived growth factor receptor (PDGFR), respectively. cDNA cloning revealed a predicted 148-kilodalton transmembrane polypeptide with structural features identifying it as a third member of the erbB/epidermal growth factor receptor (EGFR) family, prompting us to designate the gene as erbB-3. Its predicted protein structure includes an extracellular domain with two signature cysteine clusters, a single transmembrane, and its uninterrupted cytoplasmic tyrosine kinase domain exhibiting 81% and 83% significantly greater overall similarities to EGFR and erbB-2 products than to any other tyrosine kinase. The human erbB-3 gene was mapped by in situ hybridization to chromosome 12ql3 and shown to be expressed as a 6.2-kilobase transcript in a variety of normal tissues of epithelial and neuroectodermal origin. Markedly elevated erbB-3 mRNA levels were demonstrated in certain mammary tumor cell lines suggesting that elevated erbB-3 expression may play a role in some human malignancies.